T 2290/18 18-10-2022

Main request

1. Inventive step - Article 56 EPC

1.1 D7 in combination with the technical teaching of D20

1.1.1 D7 discloses the following features of claim 1 (the reference signs in parentheses referring to D7):

A gas turbine, comprising a compressor, which, via an air intake, inducts and compresses air, a combustion chamber, in which a fuel is combusted, using the compressed air, and a hot gas is produced, and also a turbine, equipped with rotor or a shaft with turbine blades, in which the hot gas is expanded, performing work (all implicit from e.g. col. 1, lines 17 to 19), wherein first means (30, 31; see Fig. 2; col. 2, lines 51 to 61) are provided in order to cool turbine blades (12) with compressed cooling air, wherein the first means comprise at least one separate compressor stage (cover plate 30 and impellers 31) which produces compressed cooling air independently of the (process air) compressor, and wherein the at least one compressor stage (30, 31) is integrated into the rotor (col. 2, lines 54 to 59), wherein the at least one compressor stage is designed as a radial compressor, and the at least one compressor stage is arranged directly aft of the last turbine rotor blades (12; see Fig. 2) in the flow direction, the at least one compressor stage inducts a cooling air mass flow (see arrows in Fig. 2 from "press. ambient" through the chamber containing the impellers 31 and into the blade 12), and the at least one compressor stage delivers a main mass flow of compressed cooling air to the last turbine rotor blades (12) for cooling purposes (col. 2, lines 55 to 59).

1.1.2 D7 therefore solely fails to disclose that the at least one compressor stage inducts a cooling air mass flow from a rotor-bearing plenum. This was not contested by the appellant.

1.1.3 The respondent argued that D7 implicitly disclosed the cooling air being induced from a rotor-bearing plenum since only two options for the source of the cooling air were available: either through the rotor shaft or from the rotor-bearing plenum. This argument is, however, not accepted.

1.1.4 D7 is silent as to the location from which the ambient pressure air for cooling is taken. No rotor bearing is depicted or discussed in the entire disclosure. The fact that a rotor bearing is present is clear, but that does not result in an unambiguous disclosure, even on an implicit basis, of the cooling air fed to the last stage of the turbine being induced from a rotor-bearing plenum in D7.

1.1.5 Regarding from where the cooling air mass flow is taken in D7, this is entirely unspecified, it solely being indicated that it is an ambient pressure source of air (see col. 3, lines 3 to 13 and Fig. 2). The objective technical problem to be solved based on the sole differentiating feature may thus be seen as being 'to provide a suitable source of air for cooling the last turbine stage'.

1.1.6 Contrary to the appellant's argument, an 'alternative' source of air cannot be part of the objective problem since no specific source of cooling air at all is disclosed in D7. The opposition division's proposed problem relating to avoiding a decrease in turbine efficiency is also not objective since D7 uses ambient air, rather than let-down pressurised process air, for cooling.

1.1.7 When wishing to solve the objective technical problem formulated in point 1.1.5 above, the skilled person would not consider D20 since, as also argued by the appellant in its letter of 20 July 2022, D20 firstly does not refer to cooling the last stage of the turbine (see last paragraph on page 2). Further, after having cooled the bearing part 12 (col. 6, line 56 to col. 7, line 1) and the strut 14 (col. 6, lines 44 to 47) of D20, the combined cooling air flow 17, 25 passes through the flow openings 21 and then into the gas path part 7 (col. 6, lines 48 to 50). This gas path part 7 contains the process air downstream of the last turbine stage. Consequently, D20 does not disclose cooling air being supplied to the last turbine stage and the air available at that point has already been heated up due to it already having been used to cool the bearing and struts.

1.1.8 Therefore, starting from D7 and wishing to solve the objective technical problem, the skilled person would not consider D20 as an appropriate basis of teaching for the problem to be solved, not least since it fails to disclose a suitable source of air for cooling the last turbine stage.

1.1.9 This specific argument was raised before the opposition division which noted this in the penultimate paragraph of point 17.2.1 of its decision. In the appeal proceedings it was explicitly raised in the last paragraph on page 2 of the appellant's letter of 20 July 2022, noting that the Board had specifically indicated in its communication under Article 15(1) RPBA 2020 (see items 2, 3 and 4) that the objective problem to be solved had to be established. The respondent was thus given adequate opportunity to comment on this issue, even if it was not addressed in the Board's preliminary opinion, not least through the possibility of participation in the scheduled oral proceedings before the Board but, in electing not to attend these, had not taken advantage of this.

1.1.10 The subject-matter of claim 1 thus involves an inventive step when starting from D7 and combining the technical teaching of D20 with this. The issue, contested by the appellant, whether D20 even disclosed a rotor bearing plenum at all in the area/volume as indicated by the respondent e.g. in its reply of 3 June 2022 (see Figures 1 and 2 thereof), therefore did not need to be answered.

1.2 D7 alone

1.2.1 In as far as the respondent might have been making an inventive step attack based on D7 alone (see its reply to the grounds of appeal, third to last paragraph on page 5, where it states that the subject-matter of claim 1 lacked an inventive step based on D7), this inventive step objection is unsubstantiated, since no indication is given of how the skilled person would actually reach the claimed subject-matter. Instead, the arguments possibly leading to this statement only take issue with certain statements made by the appellant. If the respondent's contention were even regarded to implicitly be considering the problem posed by the opposition division, this is, as found in point 1.1.6 above, not objective such that a valid inventive step objection cannot be recognised.

1.2.2 In its further letter of 3 June 2022, the respondent expanded slightly on its inventive step objection based on D7 alone, arguing that an 'alternative source of cooling air' could not be the problem to be solved since there was no alternative to inducing cooling air from the rotor-bearing plenum, other than taking this through the rotor itself. This argument is not accepted. Firstly it is noted that these are not the sole two options available to the skilled person. In addition to inducing cooling air from the rotor bearing plenum or through the rotor (as in D8, see Fig. 4; AF01 to AF04), it could alternatively be guided to the rotor blades via a path external to the rotor shaft (such as in D9, see particularly paragraph [0056], Fig. 2) or even via pipes feeding the cooling air from outside the turbine. However, irrespective of this, the respondent has failed to cite a single document in which the cooling air used for cooling the last turbine stage is induced from a rotor bearing plenum, such air being particularly suited for cooling since the air in a bearing plenum is typically slow moving and at low/atmospheric pressure. It thus follows that, while the skilled person indeed could have modified D7 by taking the cooling air from the rotor bearing plenum, the respondent has failed to provide a persuasive argument as to why the skilled person would do so. Consequently, the modification of D7 to provide cooling air from the rotor bearing plenum to the last turbine stage is not obvious to the skilled person when considering D7 alone or even when considering D7 in the light of common general knowledge.

1.2.3 Summarising, the Board finds the inventive step attack based on D7 alone to be unsubstantiated. Even if the attack were considered in substance, it is found not to be persuasive.

1.3 In conclusion the Board finds the subject-matter of claim 1 to involve an inventive step (Article 56 EPC).

2. Remittal according to Article 111(1) EPC

2.1 According to Article 111(1) EPC 1973, when deciding on an appeal, the Board may either exercise any power within the competence of the department which was responsible for the decision appealed or remit the case to that department for further prosecution.

2.2 In view of the now much narrower subject-matter in claim 1 relative to that granted, the Board exercised its discretion to remit the case to the opposition division to adapt the description to the amended claims with requisite care. A particular thing to consider relates to claim 1 having been delimited in a two-part form with respect to D7 such that, in this particular case, and in view of the large number of features of claim 1 disclosed by D7, the opposition division should consider the appropriateness of citing this document in the description.